1. Field of Invention
The invention relates to a polymer composition which is crosslinkable and suitable for crosslinked polymer applications. Furthermore the invention relates to a crosslinkable or subsequently crosslinked article, which is preferably a cable or wire, comprising said polymer composition and to a process for preparing said crosslinkable article, preferably said crosslinkable cable or wire, as well as to a subsequent crosslinking step of said crosslinkable article, preferably said crosslinkable cable or wire.
2. Description of the Related Art
Crosslinking of polymers, e.g. polyolefins, substantially contributes to an improved heat and deformation resistance, creep properties, mechanical strength, chemical resistance and abrasion resistance of a polymer. Therefore crosslinked polymers are widely used in different end applications, such as wire and cable (W&C) applications.
Electric cables and wires are generally composed of one or more polymer layers extruded around an electric conductor. In medium (between 6 kV to 36 kV) and high voltage (higher than 36 kV) power cables, the electric conductor is usually coated first with an inner semi-conducting layer, followed by an insulating layer and an outer semi-conducting layer. To these layers, further layer(s) may be added, such as screen(s) or auxiliary barrier layer(s), e.g. one or more water barrier layer(s) and one or more jacketing layer(s).
Due to above mentioned benefits achievable with crosslinking the insulating and semi-conducting layers in a cable are typically made using a crosslinkable polymer composition. The polymer composition in the formed layered cable is then crosslinked.
Common polymeric materials for wire and cable applications comprise ethylene homo- and/or copolymers (PE) and propylene homo- and/or copolymers (PP). Crosslinkable low density polyethylene (LDPE) is today one of the predominant cable insulating materials for power cables.
Crosslinking can be effected with crosslinking agents which decompose generating free radicals. Such crosslinking agents, like peroxides, are conventionally added to the polymeric material prior to or during extrusion of the cable. Said crosslinking agent should preferably remain stable during extrusion step performed at a temperature low enough to minimize the early decomposition of the crosslinking agent, but high enough to obtain proper melting and homogenisation. If a significant amount of crosslinking agent, e.g. peroxide, already decomposes in the extruder, thereby initiating premature crosslinking, this will result in the formation of so-called “scorch”, i.e. inhomogeneity, surface unevenness and possibly discolouration in the different layers of the resultant cable. Therefore, any significant decomposition of free radical forming agents during extrusion should be avoided and the crosslinking agent should decompose merely in a subsequent crosslinking step at elevated temperature. The elevated temperature increases the decomposition of the crosslinking agent and thus increases both crosslinking speed and crosslinking efficiency.
Moreover, to enable cable producers to have a high productivity in cable production lines the melt temperature of the insulation material is of importance. A slight increase in the melt temperature leads to a significant reduction in process running time and also increases the risk of scorch formation. The melt temperature can be reduced by increasing melt flow rate (MFR) of the polymer material. At the same time the flowability of the material increases which contributes to an improved processability and higher extrusion speed. A polymer with increased MFR (i.e. less viscose with lower viscosity value) would enable to increase the out put, to reduce melt pressure or to reduce melt temperature, in any combination thereof, if desired. All these parameters would also have a positive impact on the scorch performance of the material.
However, too flowable polymer layer material with high MFR will result in a non-centric cable which is not acceptable. This so called sagging brings in practice a limitation to a usable MFR of a polymer layer material, particularly in case of insulation layers.
Also the used cable production line brings limitations to the usable MFR of a polymer layer material. To avoid the undesirable sagging problem in horizontal (for example the MDCV line) and catenary continuous vulcanization (CV) lines (especially for thicker constructions) for producing a cable, it is typically required to use polymer materials, particularly for an insulation layer, which have lower MFR compared to MFR of polymer layer materials used in vertical cable production CV line and catenary continuous vulcanization (for thinner constructions). All the three cable production line types are well known in the field and described in the literature.                In a horizontal system the conductor can sink in the insulation resulting in an eccentricity of the cable core.        In a catenary CV line when the wall thickness becomes too large as the soft molten polymer mass can drop of the conductor and result in a downward displacement of the insulation layer (a so called pear shaped cable core).        
Normally these types of sagging can be counteracted by:                the use of insulation compounds of lower MFR (e.g. a more viscous material)        use of eccentric tools in the head to compensate for the effect of sinking        twisting of the cable core so that displacement of the conductor not only takes place in one direction        To counteract the second type of sagging also a double rotating technique can be used        Use of so-called entry heat treatment (EHT).        